Method of examining a substrate and corresponding device

ABSTRACT

A method of examining a substrate is provided. The method may include: generating a temperature gradient along a surface of the substrate; detecting a heat radiation emitted from the substrate; and determining as to whether the substrate is damaged based on the detected heat radiation.

RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.14/042,771, filed on Oct. 1, 2013, entitled “METHOD OF EXAMINING ASUBSTRATE AND CORRESPONDING DEVICE”, and which is incorporated herein byreference.

TECHNICAL FIELD

Various embodiments relate generally to a method of examining asubstrate and to a device for examining a substrate.

BACKGROUND

Some wafers may include defects, for example hairline cracks, othertypes of cracks, and voids, on a portion of the wafer, which are likelyto make at least a semiconductor component formed on this part of thewaver useless. The wafers may therefore be inspected in order toidentify the defects, such that at least the respective semiconductorcomponent can be discarded.

The identification of the defects may employ scanning acousticmicroscopy (SAM, also referred to as acoustic micro-imaging (AMI)) usingultrasonic waves. It may represent a suitable method to analyze materialproperties or material change, and also to detect the defects, becauseit reacts strongly to interfaces between a solid material and a gas. Itmay be a non-destructive evaluation method commonly used in failureanalysis.

FIG. 1 shows a SAM system 100 using ultrasonic waves 116 for detectingcracks 120 in a wafer 118. The system 100 serves as a sender 102 of theultrasonic waves 116 emitted by a transducer 110 through a lens 112, andas a receiver 106 of ultrasonic waves reflected by the sample 118entering the transducer 110 through the lens 112, with a switch 104switching between sender and receiver. A frequency of the ultrasonicwaves 116 may be in a range from about 5 MHz to about 500 MHz.

Ultrasonic waves can only be transmitted through solid state materialsand liquids (there is no sound wave propagation in vacuum, for example).The ultrasonic waves may be transmitted from the transducer 110 to thesample 118 to be tested via a coupling medium 114. The coupling mediummay for example be water 114, since there is almost no damping of theultrasonic waves in water. As shown in FIG. 1, both the sample 118 andthe transducer 110 may be placed in water 114, which means that theSAM-analysis may not be a contactless measurement technique.

A resolution of an acoustic image obtained by the SAM system 100 maydepend on several factors, such as the frequency of the ultrasonic wavesemitted by the transducer, focal length, numerical aperture, fluid pathand signal strength.

FIG. 2 shows an impact of ultrasonic waves 116 on different defects 226in a sample 118. Inside the sample 118, the ultrasonic waves 116 may bereflected, scattered, or absorbed.

Another conventional system includes an ultrasound system for detectingcracks in a sample.

FIG. 3 shows images 300 obtained by an SAM ultrasound system, forexample by the system shown in FIG. 1 and FIG. 3, showing samples 118with cracks 120. On the left side, a full view of the wafer is shown.All hairline cracks may be detected. Smallest details may be identified,as can be seen in the zoomed-in views of the cracks 120 shown on theright. Black points indicate particles on the wafer, caused by themeasurement tool being installed outside the clean-room.

As can be seen in the bottom row of the images, also so-called starcracks 120, which may form in a middle of the wafer 118, may beidentified.

However, an analysis using a SAM ultrasound system may not be suitablefor a high production rate, because an average time required foranalyzing one wafer 118 is about 45 minutes.

The analysis of the wafer 118 may be restricted to the edges only, whichmay save time, but then the star cracks 120 (in the bottom images inFIG. 4), which are not located at the edge of the wafer, will be missed.

SUMMARY

A method of examining a substrate is provided. The method may include:generating a temperature gradient along a surface of the substrate;detecting a heat radiation emitted from the substrate; and determiningas to whether the substrate is damaged based on the detected heatradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. The drawings are not necessarilyto scale, emphasis instead generally being placed upon illustrating theprinciples of the invention. In the following description, variousembodiments of the invention are described with reference to thefollowing drawings, in which:

FIG. 1 shows a system using ultrasonic waves for detecting cracks in asample;

FIG. 2 shows an impact of ultrasonic waves on different defects in asample;

FIG. 3 shows images obtained by an ultrasound system showing sampleswith cracks;

FIG. 4 shows an optical image and a thermography image of a switch box;

FIG. 5 shows various methods of performing thermography;

FIG. 6 shows a device for examining a substrate in accordance withvarious embodiments;

FIG. 7 shows a graph of temperature decline of wafers as a function oftime;

FIG. 8 shows into which parts a total energy that hits a substrate isdivided; and

FIG. 9 shows a setup for a method of examining a substrate in accordancewith various embodiments;

FIG. 10 shows images of a substrate obtained using a method of examininga substrate in accordance with various embodiments;

FIG. 11 shows images of a substrate obtained using a method of examininga substrate in accordance with various embodiments;

FIG. 12 compares an image of a substrate obtained by means of a methodusing ultrasound with an image of a substrate obtained using a method ofexamining a substrate in accordance with various embodiments; and

FIG. 13 shows a schematic diagram of a method of examining a substrate.

DESCRIPTION

The following detailed description refers to the accompanying drawingsthat show, by way of illustration, specific details and embodiments inwhich the invention may be practiced.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration”. Any embodiment or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or designs.

The word “over” used with regards to a deposited material formed “over”a side or surface, may be used herein to mean that the depositedmaterial may be formed “directly on”, e.g. in direct contact with, theimplied side or surface. The word “over” used with regards to adeposited material formed “over” a side or surface, may be used hereinto mean that the deposited material may be formed “indirectly on” theimplied side or surface with one or more additional layers beingarranged between the implied side or surface and the deposited material.

Infrared radiation, which is invisible to the human eye and also largelyinvisible to common detectors in cameras obtaining images in a visiblewavelength range, may be made visible using thermography.

Thermography may detect thermal radiation (e.g. black-body radiation)from an object without contacting the object. Of particular interest maybe infrared radiation in a wavelength range between approximately 1 μmand 15 μm, for example between 2 μm and 14 μm, emitted by objects havingtemperatures in a range from about 0° C. to about 800° C., for examplearound room temperature.

A detector for the detection of the thermal radiation may be atwo-dimensional array of detector elements (also called pixels, like inthe case of visual cameras, but made from a different material, forexample cadmium mercury telluride, CMT). Measured values may then bedepicted in an image, for example as a greyscale or as a false-colorimage, in which different intensity ranges are assigned differentcolors. The measured values may be converted to temperatures using theStefan-Boltzmann law, according to which the power emitted by the objectdepends on its temperature.

Several sequential images may be taken in order to obtain a video, forexample for registering a wavelike propagation of a region of elevatedtemperature (also referred to as heat wave) across the surface of theobject.

Thermography may be executed in a passive way that is suitable forrecording temperature profiles of surfaces of objects in a stationary,dynamic equilibrium with the environment. “Passive” means that surfacetemperatures of objects are measured by an infrared camera without anyexternal influence on the object. Only thermal radiation originatingfrom heat that is present in the object anyhow and a variation thereofover a surface of the object is detected. Heat sources may for examplebe internally generated heat, e.g. a heating system in a house, ornatural heat sources like solar radiation. Also cooling may affect thethermal radiation emanating from the surface of the object, for examplecooling by wind.

A thermal image (also referred to as thermogram) of an object may berecorded by means of the infrared camera. The thermogram may be analyzedaccording to its temperature distribution. Thermal abnormalities may bedetected in this way. Such thermal abnormalities may for example befound in electrical connections or devices. FIG. 4 shows an image 430 ofa fuse box 432 taken at visual wavelengths on the left, and a thermogram434 of the same fuse box 432 on the right. A faulty fuse on the leftside of the box and its connecting wires show up darker than the otherfuses and wires in the thermogram. Other fields where passivethermography may be applied include thermography of insulated houses inorder to detect thermal bridges and thermography of under floor heatingsystems to detect leakage. For such typical applications of the passivethermography, reflective radiation such as sunlight usually represents astrong disturbance, and it may be required to keep its contribution to atotal infrared radiation from the object small.

For different kinds of applications, thermography may also be executedin an active way. This means that the object to be analyzed is excitedthermally, and then its response behavior is registered andinvestigated. In other words, active thermography causes an intendedchange of the surface temperature to be measured. For this, a steadystate of an object may be changed, so that the resulting temperaturebehavior can be used for characterization.

Heat sources such as lasers or lamps may be used for the thermographictesting to produce a heat flow inside the object. The object may beexcited one-time (so-called impulse thermography), periodically (forexample sinusoidally, so-called lock-in-thermography), or continuously.This may lead to a direct temperature change (for example by lamps) orto an indirect temperature change (for example by ultrasonic). In caseswhere such a temperature change propagates in a wave like manner, it mayalso be referred to as thermal wave, temperature wave or heat wave. Thegenerated temperature change of the object may be measured by detectingand analyzing the infrared radiation from the surface of the object.Detection may be performed for example on a point-like region, on anentire image, or by obtaining a sequence of images (a film).

A variation of object properties in or slightly beneath an area of theimaged surface, for example a different material or a crack, affects,i.e. disturbs, the thermal radiation from that surface area. It may forexample have a different temperature than a surface region surroundingit, or its temperature variation may occur time-shifted.

By means of the described disturbance of the heat flow, activethermography, with thermal excitation applied from the outside one-timeor periodically as described above, may make an identification ofdefects in or near the examined surface possible. Such defects may forexample include hairline cracks in/on or near the surface.

The following table provides a brief comparison between passive andactive thermography:

Active Thermography Classification Passive Thermography Lock-in methodImpulse method Excitation Internal heat Modulated laser, Flash lamp,mode Infrared radiator Infrared radiator Temperature IR camera IRdetectors IR camera measurement Sonic detectors Liquid crystals Testingtime Some seconds Some minutes Some seconds Result Single image Singleimage Film sequence

Detection of the infrared radiation, in particular if two dimensionalimages are to be obtained, may be performed using an infrared camera.This may for example be a so-called focal plane array (FPA) infraredcamera, including for example a cadmium mercury telluride (CMT) infrareddetector, which may mainly be used in a wavelength range between 3 μmand 14 μm. The detector may include a two-dimensional array of pixelsconverting an infrared radiation intensity radiated onto any given pixelto a numerical value, thereby creating a two-dimensional map ofnumerical values. Said numerical values may be used directly, or thenumerical values may be converted to temperature values, therebycreating a two-dimensional temperature map. The two-dimensional map ofnumerical values or temperature values, may be displayed directly as atwo-dimensional image, for example on a display of the infrared cameraor on a display of a computer, or it may be stored, for example in amemory of the infrared camera or a memory of the computer, for futureuse, for example analysis or inspection.

Applications that do not require two-dimensional images may usedifferent kinds of infrared detectors. For example a pyrometer (alsoreferred to as radiation thermometer) may be used for detecting infraredradiation from a point-like region, or an infrared line camera (alsoreferred to as IR scan camera) may be used in applications where a lineis to be imaged, for example when production processes are monitored.

A schematic representation of different kinds of infrared detectors isshown in FIG. 5: a pyrometer on the left in panel 536, a scan camera inthe middle in panel 538, and a focal plane array (FPA) camera on theright in panel 540.

FIG. 6 shows a device 600 for examining a substrate 648 in accordancewith various embodiments. The device 600 may be used for applying themethod of examining the substrate 648 according to various embodimets.In various embodiments, the device 600 may include a holder 650, a heatsource 642, and a detector 644. A substrate 648 may be provided. Thesubstrate 648 may include a main processing surface (which may also bereferred to as a first surface) 656 and a second surface 658 oppositethe main processing surface 656. A size of the main processing surface656 and/or the second surface 658 in a first direction may be denoted A,and a thickness of the substrate 648, i.e. a size of the substrate 648between the main processing surface 656 and the second surface 658 maybe denoted T. In various embodiments, the substrate 648 may include orconsist of at least one material from a group of materials, the groupincluding or consisting of: a semiconductor material, for examplesilicon, silicon carbide, gallium arsenide or indium phosphide, adielectric material, for example a glass or a plastic material, and anelectrically conductive material, for example a metal or a metal alloy.In various embodiments, the substrate 648 may be a wafer, for example athinned wafer. In various embodiments, the substrate 648 may be anembedded wafer. In various embodiments, the size A of the substrate 648may be in a range from about 50 mm to about 1000 mm, for example fromabout 50 mm to about 500 mm, for example from about 100 mm to about 450mm. In various embodiments, the thickness T of the substrate 648 may bein a range from about 30 μm to about 10 mm, for example from about 40 μmto about 1 mm, for example from about 40 μm to about 70 μm.

In various embodiments, the substrate 648 may be arranged on at leastone holder 650. The holder 650 may be configured in such a way that onlya small amount of heat is conducted from the substrate 648 throughand/or into the holder 650. The holder 650 may for example include orconsist of a material with low thermal conductivity, for example aplastic material or glass. The holder 650 may be shaped in such a waythat a contact area with the substrate 648 is small, for example theholder 650 may be contacting the substrate 648 with a point, a taperededge, a narrow rim, or the like.

In various embodiments, the method of examining a substrate 648 mayinclude generating a temperature gradient across the surface of thesubstrate 648. The temperature gradient may be time variable, forexample it may propagate across the surface of the substrate 648 overtime. It may propagate in a wave-like manner. It may propagate in thedirection of the temperature gradient. The setup 600 may be used forgenerating the temperature gradient across the surface of the substrate648. In various embodiments, the setup 600 may include a heat source642. The heat source 642 may be used for heating at least a part of thesubstrate 648. In other words, the substrate 648, or at least a part ofit, may be excited by the heat source 642. In various embodiments, theheating of at least a part of the substrate 648 may generate thetemperature gradient along the surface of the substrate 648. In otherwords, the temperature gradient may be generated by heating the part ofthe substrate 648 by the heat source 642 directly (“directly heated” inthis case is to be understood as being excited by the heat provided bythe heat source 642, e.g. by its thermal radiation 652, as opposed toindirect heating through thermal conduction from directly heated parts),while not heating other parts of the substrate 648. Those parts may beindirectly heated by thermal conduction from the part of the substrate648 that was heated by the heat source 642. The temperature gradient maybe formed between parts of the substrate 648, e.g. parts of the surfacewith high temperature heated by the heat source 642, and parts of thesubstrate 648, e.g. parts of the surface, with low temperature not (atleast not directly) heated by the heat source. Heat conduction may occurin the substrate 648, and thus the heat may spread and/or move acrossthe surface of the substrate 648, for example in a wave-like manner.This may be considered as indirect heating of parts of the substrate 648by the heat source 642. In various embodiments, a temperature differencebetween the high temperature and the low temperature may be in a rangefrom 1 mK to 10 K, for example from 2 mK to 1 K. In various embodiments,the surface 656 or 658 along which the temperature gradient is generatedmay be the same as the parts of the substrate 648 heated by the heatsource 642. This may be referred to as reflection method (shown in FIG.7), because then the heat source 642 and a detector 644 that may beprovided for detecting thermal emission from the elevated temperatureparts of the surface 656 or 658 are arranged on the same side of a planedefined by the surface 656 or 658, respectively. In various embodiments,the surface 656 or 658 along which the temperature gradient is generatedmay be different from the parts of the substrate 648 heated by the heatsource 642, e.g. the other surface 658 or 656. This may be referred toas transmission method, because then the heat source 642 and thedetector 644 that may be provided for detecting thermal emission fromthe elevated temperature parts of the surface 656 or 658 are arranged ondifferent sides of the plane defined by the surface 656 or 658,respectively. In various embodiments, the temperature gradient may begenerated within the part of the substrate 648 not directly heated bythe heat source 642. The temperature gradient may be formed betweenparts of the substrate 648, e.g. parts of the surface, with highertemperature, that have received, through thermal conduction, more of theheat that had been transferred by the heat source 642 to a differentpart of the substrate 648, and parts of the substrate 648, e.g. parts ofthe surface, with lower temperature that have received, through thermalconduction, less of the heat that had been transferred by the heatsource 642 to a different part of the substrate 648. In variousembodiments, the temperature difference between the high temperature andthe low temperature may be in a range from 1 mK to 10 K, for examplefrom 2 mK to 1 K. In various embodiments, the heat source 648 may forexample be arranged above the plane defined by the surface 656, belowthe plane defined by the second surface 648, or in one or both of theplanes defined by the surface 656 and the surface 658, respectively.

In various embodiments, the heat source 642 may be used for heating thecomplete surface or the complete substrate 648 or a large fraction ofthe surface or the substrate 648, e.g. more than about 20%, directly. Atleast in those various embodiments, where at least a large fraction ofthe surface or the substrate 648 is directly heated by the heat source642, a temperature gradient may be generated within the part of thesurface heated by the heat source 642. This may be achieved bytransferring more heat to some regions within the part of the substrate648 directly heated by the heat source 642 than to other regions withinthe part of the substrate 648 directly heated by the heat source 642.This may, in a case of a radiative heat source 642, for example beachieved by arranging the heat source 642 in such a way that a distancefrom a point or region of the heat source 642 where heat is emitted tothe substrate 648 is shorter for some of the directly heated regions ofthe substrate 648 than for others. In various embodiments, a radiationdirection 660 may enclose an angle different from 0° with respect to anormal 662 on the surface. In other words, the radiation direction 660may have a vector component that is parallel to the surface. Theradiation direction 660 may be considered as being the direction of thecenter line of a radiation cone formed by the radiation. In variousembodiments, this may be achieved by arranging the heat source 642 insuch a way that the radiation direction 660 is inclined by a non-zeroangle with respect to the normal 662 on the surface, for example like itis shown in FIG. 6. In various embodiments, the heat source 642 may bearranged at an angle of 90° with respect to the normal 662 on thesubstrate 648 and/or a center line of an aperture of the detector 644,for example an infrared camera 644. In this way, the temperaturegradient across the surface 656 or 658 may initially extend between aregion of elevated temperature on an edge of the substrate 648 and aregion of lower temperature on an opposite edge of the substrate 648.Thereafter, the region of elevated temperature may propagate across thesurface 656 or 658, e.g. in a wave-like manner. The detector, e.g. theinfrared camera, may record this behaviour. In various embodiments, thismay be achieved by varying a wavelength and/or an intensity of theradiation over the part heated by the heat source 642, for example usingone or more of a spectrally dispersive element, a lens, or a lightbaffle. In various embodiments, the temperature gradient may begenerated only within the heated part of the surface, e.g. when theheated part of the surface is the whole surface. The temperaturegradient may be formed between parts of the substrate 648, e.g. parts ofthe surface with high temperature to which more heat had beentransferred by the heat source 642, and parts of the substrate 648, e.g.parts of the surface, with low temperature to which less heat had beentransferred by the heat source 642. In various embodiments, thetemperature difference between the high temperature and the lowtemperature may be in a range from 1 mK to 10 K, for example from 2 mKto 1 K. In various embodiments, the temperature gradient may begenerated within the heated part of the substrate 648 and additionallywithin the part of the substrate 648 not directly heated by the heatsource 642. The temperature gradient may be formed between parts of thesubstrate 648, e.g. parts of the surface with high temperature to whichmore heat had been transferred by the heat source 642 and continuethrough parts of the substrate 648, e.g. parts of the surface, withlower temperature to which less heat had been transferred by the heatsource 642 to parts of the substrate 648, e.g. parts of the surface,with low temperature, to which no heat had been directly transferred bythe heat source 642. In various embodiments, the temperature differencebetween the high temperature and the low temperature may be in a rangefrom 1 mK to 10 K, for example from 2 mK to 1 K.

The surface along which the temperature gradient is generated may forexample be the main processing surface 656 or the second surface 658. Invarious embodiments, the heat source 642 may heat the part of thesubstrate 648 by means of heat conduction. In various embodiments, theheat source 642 may heat the part of the substrate 648 contactlessly. Invarious embodiments, the heat source 642 may heat the part of thesubstrate 648 by means of radiating radiation 652 towards the substrate648. The radiation 652 may be or include electromagnetic radiation, forexample electromagnetic radiation in the visible wavelength range, i.e.in a wavelength range from about 390 nm to about 700 nm, and/orelectromagnetic radiation in a wavelength range with wavelengths shorterthan the visible wavelength range, for example in the UV or EUV region,i.e. in a wavelength range from about 10 nm to about 390 nm, and/orthermal radiation, for example infrared radiation, i.e. radiation in awavelength range from about 1 μm to about 15 μm, and/or any otherradiation that is suitable for heating at least the part of thesubstrate 648, such that the heating of at least the part of thesubstrate 648 generates the temperature gradient along the surface ofthe substrate 642 without destroying the substrate 648 or the surface.The radiation 652 may be or include continuous emission, like it is forexample provided by a filament lamp, a heat lamp or a flash lamp, and/orit may be or include line emission radiation, like it is for exampleprovided by a laser.

As shown in FIG. 9, in various embodiments, the heat source 642 may heatthe substrate 648 by transferring heat to the substrate 648 for a shortduration. In other words, the heat source 642 may provide a short energyimpulse, i.e. an energy pulse for a short time (impulse thermography).This is depicted in FIG. 9 by the flash-like shape of the heat 652radiated by the heat source (e.g. the flash lamp) 642 towards thesubstrate (e.g. the wafer) 648. The temperature gradient (highertemperature depicted as darker shade) generated across the surface ofthe substrate 648 may be detected by the detector 644. The short pulseduration/time may for example be in a range from 1 ms to about 1 s, forexample from 1 ms to 5 ms, for example from 2 ms to 4 ms. In variousembodiments, the pulse duration may depend on a thermal conductivity ofthe substrate 648. The pulse duration and the thermal conductivity ofthe substrate 648 may be anti-proportional, i.e. the higher the thermalconductivity, the shorter/smaller the pulse duration. Too much energyinput (i.e. too long pulse duration, too high energy) may damage thesubstrate 648. In various embodiments, the heat source 642 may be a fastreacting radiator, a flash lamp, for example a flash lamp with theshortest possible pulse duration and a high flash intensity, or a laser.

In various embodiments, the method of examining a substrate 648 mayinclude detecting a heat radiation 654 emitted from the substrate 648.In various embodiments, the detector 644 for detecting the heatradiation 654 from the substrate 648 may be or include at least one of athermometer, a pyrometer, a bolometer, a microbolometer, a thermocouple,a thermopile, a photo detector, for example a photo detector sensitiveto near-infrared and/or infrared radiation, or an array of any of thesedetectors. In various embodiments, the detector 644 may for example beor include a camera, e.g. an infrared camera 644 including a cadmiummercury telluride infrared detector array. Such a detector array may forexample include 65,536 pixels, arranged in an array of 256 pixels by 256pixels. The infrared detector array, e.g. the infrared camera 644, maybe sensitive to infrared radiation in the wavelength range from about 3μm to about 5 μm (mid-infrared, MIR). In various other embodiments, theinfrared camera 644 may have different specifications, like a higher orlower number of pixels, for example 128×128, 512×512 or 1024×1024pixels, or it may be sensitive to a broader, narrower, or differentwavelength range, for example to a wavelength range from about 1 μm toabout 15 μm, or from about 10 μm to about 12 μm.

In various embodiments, the detector 644 may be arranged such that itcan detect the heat radiation 654 emitted from the substrate 648. It maybe arranged such that it can detect the heat radiation from at least apart of the part of the substrate 648, e.g. the part of the surface 656or 658 along which the temperature gradient was created. In variousembodiments, e.g. if the detector 644 is an optical system, for examplea camera, that includes a detector array, the detector 644 may bearranged such that the surface, or at least a part of the surface,across which the temperature gradient is generated, is focused onto afocal plane of the detector 644. In various embodiments, the detector644 may for example be arranged with an entrance pupil parallel to andfacing the surface, across which the temperature gradient is generated.In various embodiments, parameters like size A and B of the substrate648, size of the detector 644, distance between the surface 656 or 658,across which the temperature gradient is created, and the detector 644,magnification of the optical system of the detector 644, additionaloptical elements 646 and opening angle of the optical system may bechosen such that the whole surface 656 or 658 across which thetemperature gradient is created may be detected by the detector in asingle image. In other embodiments, only a part of the surface 656 or658 across which the temperature gradient is created may be detected bythe detector 644 in a single image.

For example, at a given time only a part of the surface 656 or 658currently having an elevated temperature, caused directly or indirectlyby the heat source 742, may be detected by the detector 644, for exampleby a 2-dimensional detector array 644 or a 1-dimensional detector line644.

In various embodiments, the detector 644 may detect the heat radiation654 emitted from the substrate 648 only once. In various otherembodiments, the detector 644 may detect the heat radiation 654 emittedfrom the substrate 648 multiple times, for example by sequentiallydetecting the heat radiation 654 emitted from the substrate 648. Invarious embodiments, the sequential detection of the heat radiation 654may include detecting the heat radiation 654 at a frequency that is ashigh as the detector 644 will allow. In various embodiments, an imagefrequency (also referred to as frame rate) of the detector 644, forexample the CMT array, may be 442 Hz. In various embodiments, higher orlower frequencies may be possible. In various other embodiments, thedetection of the heat radiation may be performed at a differentfrequency. A detection may be executed or a detection sequence may bestarted by a trigger event, for example by the generation of thetemperature gradient by the heat source 642, and/or by the release ofheat by the heat source 642, e.g. by the flash of the flash lamp, or bya region of interest undergoing a temperature change. In variousembodiments, if the detector 622 includes a 2-dimensional detectorarray, the sequence of images may form a movie.

In various embodiments, the flash lamp 642, for example with a flashintensity of 6000 J, may excite the substrate 648. The temperature ofthe substrate 648 may increase by some amount, for example by a fewKelvin (K), e.g. below 5 K. If necessary or useful, the warming of thesubstrate 648 may be recorded using the infrared camera 644. After thiswarming of the substrate 648, a thermal decay behavior of the substrate648 may be observed using the infrared camera 644 (see the thermal decaybehavior shown in temperature-time-diagram 700 of FIG. 7). FIG. 7 showsa first characteristic 702 illustrating a wafer 648 as the substrate648, which is in order. FIG. 7 furthermore shows a second characteristic704 illustrating a wafer 648 as the substrate 648, which includes acrack.

Observations of the thermal decay behavior of the substrate 648, e.g.impulse thermography, may be performed from a side of the substrate 648that was excited by the flash (reflection method). In other words, usingthe reflection method, the heat source 642 and the infrared camera 644may be directed at the same surface of the substrate 648, as shown inFIG. 7. In various other embodiments, the heat source 642 and theinfrared camera 644 may be directed at different surfaces of thesubstrate 648 (transmission method). In other word, in the transmissionmethod, the substrate 648 may be arranged between the heat source 642and the infrared camera 644.

In various embodiments, the method of examining the substrate 648 mayinclude determining as to whether the substrate 648 is damaged based onthe detected heat radiation 654. In various embodiments, determining asto whether the substrate 648 is damaged may include a visual inspectionof the detected heat radiation, e.g. of a detection signal. Thedetection signal may be displayed on a display of the detector 644, orit may be transferred to an external device for displaying it for visualinspection, for example an external monitor, a computer with a monitor,or the like. In various embodiments, the detection signal may be thetwo-dimensional image of the surface or a part of the surface 656 or 658across which the temperature gradient was generated. In variousembodiments, the detection signal may be a movie of such images. Thedetection signals may be stored in a storage device of the detector 644or of the external device. Based on the visual inspection, a user maydetermine whether the substrate 648 is damaged.

In various embodiments, determining as to whether the substrate 648 isdamaged may include processing the detection signal, for example byusing a processing circuit. Determining as to whether the substrate 648is damaged may include processing the detection signal of a singledetection signal or of several detection signals, for example of a timesequence, with the processing circuit. The processing circuit may forexample be configured to detect unexpected abrupt changes in thedetection signal, and/or it may be configured to compare the detectionsignal of the substrate 648 presently examined with a detection signalof an undamaged reference substrate. In various embodiments, theprocessing circuit may be configured to determine from the processeddetection signals whether the substrate is damaged. An analysis ofdetection signals according to various embodiments is shown in FIG. 7,in which time sequences of detection signals of the reference substrate(first characteristic 702 having a dashed line) and the presentlyexamined substrate (second characteristic 704 having a solid line) wereprocessed by the processing circuit to determine a thermal decaybehavior of the two substrates 648. A temperature difference indicatedby ΔT_(max) allows a conclusion that the presently examined substrate isdefective. In various embodiments, visual inspection and processing ofthe detection signal may be combined, for example by first processingthe detection signal, and then displaying the processing result forvisual inspection, for example by displaying a processed image or imagesequence, or a graph of a processing result. The user may then decidewhether the substrate 648 is damaged.

As shown in FIG. 8, in various embodiments, for a determination of thetemperature of the substrate it has to be considered that the detectedsignal, i.e. the measured energy, consists of three different sources:

1. Energy φ_(abs) that comes directly from the surface 656 or 658 of thesubstrate 648: The substrate 648 may warm up because of the absorbanceof heating energy. This heat may be radiated to the outside, and thistemperature may correspond to the effective temperature of the substrate648.

2. Energy φ_(ref) that does not originate from the substrate 648: Energyφ_(ref) may be reflected at the surface of the substrate 648. Suchreflection may falsify the temperature determination, because thereflected energy φ_(ref) may not reveal anything about the realtemperature of the substrate 648.

3. Another energy φ_(trans) that does originate from the substrate 648:Transmission energy φ_(trans) may be an energy where the radiationshines through the substrate. The energy φ_(trans) may emanate (or seemto emanate) from the substrate 648, but it may not heat up the substrate648, because it may be transmitted without being absorbed in thesubstrate 648. Also this kind of energy φ_(trans) may falsify thetemperature determination, because it may not reveal anything about thereal temperature of the substrate. In other words, as shown in FIG. 8, atotal energy φ_(tot) that hits any object may be subdivided in threeparts that may consist of absorbancφ_(abs), reflection φ_(ref) andtransmission φ_(trans). In various embodiments, only the part φ_(abs) ofthe heat φ_(heat), which forms part of the total energy φ_(tot), that isabsorbed by the substrate 648 and then re-emitted by the substrate 648may be useful for the examination of the substrate 648.

In various embodiments, it may be sufficient to generate the temperaturegradient across the surface 656 or 658 of the substrate 648 from onedirection, for example if damages in the substrate 648 are expected tooccur only in a way where they are elongated along a pre-determineddirection, for example along certain directions in a crystal lattice. Inthat case, the substrate 648 may be arranged in such a way that theexpected damage is not parallel to the thermal gradient (and itspropagation direction). In various embodiments, the method of examiningthe substrate 648 may include two complete sequences of executing themethod, including generating a temperature gradient along a surface 656or 658 of the substrate 648; detecting a heat radiation 654 emitted fromthe substrate 648; and determining as to whether the substrate 648 isdamaged based on the detected heat radiation 654; and in between the twosequences, rotating the substrate 648 around the normal on the surface656 or 658. In various embodiments, the substrate 648 may for example berotated by an angle in a range from about 45° to about 90°, for exampleby 45° or by 90°. In other words, the method of examining the substrate648 may include generating a temperature gradient along a surface 656 or658 of the substrate 648; detecting a heat radiation 654 emitted fromthe substrate 648; determining as to whether the substrate 648 isdamaged based on the detected heat radiation 654; and thereafter, forexample if it is determined from this first sequence that the substrate648 is undamaged, rotating the substrate 648 around the normal on thesurface 656 or 658; generating a temperature gradient along a surface656 or 658 of the substrate 648; detecting a heat radiation 654 emittedfrom the substrate 648; and determining as to whether the substrate 648is damaged based on the detected heat radiation 654. In variousembodiments, the first determining as to whether the substrate 648 isdamaged may be deferred to later, after the second generating of thetemperature gradient along the surface of the substrate 648 and thesecond detecting of the heat radiation emitted from the substrate 648.

FIG. 10 shows images 1000 of a substrate 648 obtained using a method ofexamining a substrate 648 in accordance with various embodiments. Thetemperature gradient was generated across the surfaces shown in theimages 1000. As depicted in FIG. 10, the heat source 642 for generatingthe temperature gradient was located to the left of the substrates 648,and its heat was directed at the shown surfaces of the substrates 648.The temperature gradient initially showed its region of highesttemperature on the left or leftmost edge of the surface, and its regionof lowest temperature on the right or rightmost edge of the surface.Thereafter, the heat, i.e. the region of highest temperature propagatedover the surface of the substrate 648 from the left to the right, forexample in a wave-like manner, like a heat wave. The heat radiationemitted from the substrate was detected by a detector (not shown), inthis case by an infrared camera (not shown) directed at the surface. Theinfrared camera recorded the images (thermograms) 1000. The images 1000represent individual images 1000 from a sequence of images taken of thesurface during the propagation of the heat wave. The image on the leftwas obtained with the substrate 648 at an orientation designated as 0°.The image on the right of the same surface of the same substrate 648 wasobtained after having rotated the substrate by 90°, in this caseclockwise, around the normal on the surface. In the image on the left,the damage (a crack or hairline crack) 1064 in the circled region isbarely detectable (it may be identified if its location is knownbeforehand). In the image on the right, the same damage 1064 is clearlydetectable as a bright line-shaped region in the image. Damages 1064with their long axis oriented parallel to the direction of thetemperature gradient, i.e. the direction of heat propagation, in otherwords, pointing towards the heat source 642, may be more difficult todetect than damages 1064 oriented such that their long axis encloses anangle with the direction of the temperature gradient. Damages 1064 maybe easiest to detect if their long axis encloses an angle of 90° withthe direction of the temperature gradient. For detecting the damage1064, images 1000 may have been selected in which the damage 1064 showsup best. For example, in images taken at a time when the heat wave hasnot yet reached the region with the damage, or when some time hasalready passed since the passage of the heat wave, the damage 1064 maynot show up in the image. Bright dots in the images 1000 may be ignored,because they represent artifacts caused by pixel errors of the infraredcamera 644. If a pixel is saturated, it usually shows up in white,defective pixels in black, but sometimes also in gray. Usually, this maybe corrected by calibration of the individual pixels, or it may beprevented to some extent by careful selection of camera setupparameters.

FIG. 11 shows images (thermograms) 1100 of a substrate 648 obtainedusing a method of examining a substrate in accordance with variousembodiments. The images 1100 were obtained in a similar manner to theimages shown in FIG. 10, the difference being that the substrate 648 isdifferent, and that the damage 1064 extends in a direction that shows upwith the substrate 648 oriented at 0° (shown in the left of the image1100). With the substrate rotated by 90° clockwise around the normal onthe surface (shown in the right of the image 1100), the damage 1064 ishardly detectable.

FIG. 12 compares an image 1200 a of a substrate 648 obtained by means ofa method using ultrasound (SAM-image) with an image 1200 b of thesubstrate 648 obtained using a method of examining the substrate inaccordance with various embodiments. The image 1200 b is identical tothe image shown on the left in FIG. 11. As the comparison shows, thedamage 1064 detected in the image 1200 b can also be identified in thecomparison image 1200 a. Further comparisons of other substrates (notshown) showed that nearly every damage (hairline crack) was identifiedwith standard settings. Modifications of the settings may be employedfor optimization of the method.

FIG. 13 shows a schematic diagram 1300 of a method of examining asubstrate 648. The method may include: generating a temperature gradientalong a surface of the substrate (in 1310); detecting a heat radiationemitted from the substrate (in 1320); and determining as to whether thesubstrate is damaged based on the detected heat radiation (in 1330).

In various embodiments, the method of examining a substrate may providea method that requires only a short time, for example less than 10seconds, for examining a substrate.

In various embodiments, the method of examining a substrate may providea fast and effective testing method that provides results of detectingdamages in the substrate. In various embodiments, active thermography,for example impulse thermography, may be used. In various embodiments,damages/defects may be detected in a substrate without a prioriknowledge regarding a presence and/or nature of a damage. In variousembodiments, the method may be a photo-thermal measurement procedure,which means that the substrate may be excited by a heat source, and heatradiation from the substrate may be detected by a photo detector.

In various embodiments, a method of examining a substrate is provided.The method may include: generating a temperature gradient along asurface of the substrate; detecting a heat radiation emitted from thesubstrate; and determining as to whether the substrate is damaged basedon the detected heat radiation.

In various embodiments, the surface may include at least one of a mainprocessing surface of the substrate and a surface of the substrateopposite the main processing surface. In various embodiments, thesubstrate may be a wafer. In various embodiments, the temperaturegradient may be time variable. In various embodiments, the temperaturegradient may be formed contactless. In various embodiments, thetemperature gradient may be formed by radiating heat. In variousembodiments, the heat may be radiated in a direction that has a vectorcomponent at an angle to a normal on the surface. In variousembodiments, the heat may be radiated in a direction that has a vectorcomponent parallel to the surface. In various embodiments, forming thetemperature gradient may include applying heat to a small part of thesubstrate. In various embodiments, the heat may be applied with a laser.In various embodiments, the heat may be applied for a short duration. Invarious embodiments, the temperature gradient may span a temperaturedifference of less than 5 K. In various embodiments, detecting a heatradiation emitted from the substrate may include detecting infraredradiation emitted from the substrate. In various embodiments, thedetected infrared radiation may have a wavelength in a range from 2 μmto 15 μm. In various embodiments, detecting a heat radiation emittedfrom the substrate ma include forming an image of at least part of thesubstrate. In various embodiments, detecting a heat radiation emittedfrom the substrate may include detecting the heat radiation with aninfrared camera. In various embodiments, determining as to whether thesubstrate is damaged based on the detected heat radiation may includeanalyzing the image. In various embodiments, determining as to whetherthe substrate is damaged based on the detected heat radiation mayinclude at least one of analyzing the detected heat radiation visuallyand analyzing the detected heat radiation using a processing circuit. Invarious embodiments, the method may further include: before generatingthe temperature gradient along the surface of the substrate, generatinga temperature gradient along a surface of the substrate; detecting aheat radiation emitted from the substrate; and rotating the substrate byat least 45° around a normal to the surface.

In various embodiments, a device for examining a substrate is provided.The device may include: a holder; a heat source arranged relative to thesubstrate holder and configured to generate a temperature gradient alonga surface of the substrate held in the substrate holder; and a detectorconfigured to detect a heat radiation emitted from the substrate held bythe substrate holder.

In various embodiments, the heat source may be configured to emit heatradiation. In various embodiments, the heat source may be arranged suchthat the heat radiation includes a radiation direction that has a vectorcomponent parallel to the surface of the substrate. In variousembodiments, the substrate holder may include or consist of a materialwith low thermal conductivity. In various embodiments, the substrateholder may be configured to contact the substrate with a point or anarrow rim. In various embodiments, the heat source may be configured todirectly provide heat only to a part of the substrate held by thesubstrate holder. In various embodiments, the detector may be configuredto detect infrared radiation. In various embodiments, the detector maybe an infrared camera. In various embodiments, the substrate may be awafer.

While the invention has been particularly shown and described withreference to specific embodiments, it should be understood by thoseskilled in the art that various changes in form and detail may be madetherein without departing from the spirit and scope of the invention asdefined by the appended claims. The scope of the invention is thusindicated by the appended claims and all changes which come within themeaning and range of equivalency of the claims are therefore intended tobe embraced.

What is claimed is:
 1. A system for examining a substrate, the systemcomprising: a substrate holder; a heat source arranged relative to thesubstrate holder and configured to generate a temperature gradient byradiating heat to all of a main processing surface of a substrate heldin the substrate holder; and a detector configured to detect a heatradiation emitted from the substrate held in the substrate holder. 2.The system of claim 1, wherein the heat source is arranged so that theheat radiation comprises a radiation direction that has a vectorcomponent parallel to the main processing surface of the substrate heldin the substrate holder.
 3. The system of claim 1, wherein the substrateholder comprises or consists of a material with low thermalconductivity.
 4. The system of claim 1, wherein the substrate holder isconfigured to contact the substrate with a point or a narrow rim.
 5. Thesystem of claim 1, wherein the detector is configured to detect infraredradiation.
 6. The system of claim 1, wherein the detector comprises aninfrared camera.
 7. The system of claim 1, wherein the detectorcomprises a thermometer, a pyrometer, a bolometer, a microbolometer, athermocouple, a thermopile, and/or a photo detector.
 8. The system ofclaim 6, wherein the infrared detector comprises a cadmium mercurytelluride infrared detector array.
 10. The system of claim 6, whereinthe infrared camera is sensitive to infrared radiation in the wavelengthrange from about 3 μm to about 5 μm.
 11. The system of claim 1, whereinthe heat source comprises a fast reacting radiator.
 12. The system ofclaim 1, wherein the heat source comprises a laser.
 13. The system ofclaim 1, wherein the heat source comprises a flash lamp.
 14. The systemof claim 13, wherein the flash lamp is configured to provide a flashintensity of 6000 J.
 15. The system of claim 1, further comprising aprocessing circuit configured to analyze the detected heat radiation.